This invention relates to an improvement in a dither circuit employed for improving linearity in analog-to-digital(A/D) or digital-to-analog (D/A) conversion of a signal and, more particularly, to a dither circuit capable of preventing occurrence of residual dither component in an output signal from an A/D or D/A converter in a case where clipping has occurred in the level of an input signal to be converted.
In a quantizing operation in A/D conversion, partial missing of information in the quantized output signal is inevitable because continuously distributed sampled values in an input signal are expressed by dispersed values in the quantized output signal. This difference between the quantized output signal and the input sampled values becomes a quantizing noise which is an inherent restriction in the digita signal processing. As is well known, the quantizing noise becomes a white noise which is uncorrelated with a signal to be converted if the signal is of a large level whereas the quantizing noise becomes distortion which is greatly correlated with the signal if the signal is of a relatively small level and this noise adversely affects the quality of tone when the signal is sounded in an audio apparatus.
For overcoming this problem, employment of a dither circuit with an A/D converter is proposed. The dither circuit is provided for uncorrelating quantizing noise with a signal to be converted by superposing dither (random noise) for LSB (least significant bit) one bit on an input signal before A/D conversion and subtracting dither component from a digital signal after the A/D conversion. Further, for uncorrelating not only quantizing error but also conversion error caused by, e.g., manufacturing error in an A/D converter and a D/A converter, a proposal is made for imposing a large amplitude dither which consists of random noise of integer multiple of LSB one bit on an input signal before A/D or D/A conversion and subtracting dither component from a converted output.
A general construction of a dither circuit is shown in FIG. 2. In FIG. 2, an input analog signal is A/D converted and its output digital signal is D/A converted to an analog signal again.
In FIG. 2, a dither generator 10 outputs dither (random noise) in digital. Dither is converted to an analog signal by a D/A converter 12 and thereafter is superposed on an input analog signal by an adder 14. The input signal superposed with dither is converted to a digital signal by an A/D converter 16. Dither component is subtracted from the converted digital signal by a subtractor 18 whereby a digital signal corresponding to the input analog signal is derived.
Dither is superposed on an input digital signal to a D/A converter 22 by an adder 20. The signal superposed with dither is converted to an analog signal by a D/A converter 22. Dither is converted to an analog signal by a D/A converter 24 and subtracted from an output analog signal from the D/A converter 22 by a subtractor 26 whereby an analog signal corresponding to the input digital signal is derived.
In a dither circuit, for sufficiently achieving the dither effect of uncorrelating quantizing error and conversion error effectively, a relatively large level of dither is required. When the input signal has reached a level at which clipping occurs, a simply clipped output signal is produced as shown in FIG. 3 if there is no dither applied to the input signal. When, however, the input signal has reached a level at which clipping occurs in a state in which dither has been superposed on the input signal, an accurate subtraction of dither from the output signal cannot be made with a result that dither component remains in the output signal as shown in FIG. 4 and is heard as noise. Even if the level of the input signal itself has not reached a clipping level, the level of the input signal superposed with dither will reach the clipping level if the level of dither is large enough and the above described problem will likewise arise.
It is, therefore, an object of the invention to provide a dither circuit capable of preventing remaining of dither component in a converted output signal in A/D or D/A conversion due to reaching of an input signal superposed with dither to a clipping level when the level of the input signal is relatively large.